Operations and Significant Accounting Policies - Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Lessee, Lease, Description [Line Items]
Lessee, Operating Lease, Residual Value Guarantee, Description	Our largest operating lease is for the dragline at BNI Energy which includes a termination payment at the end of the lease term if we do not exercise our purchase option.
Operating Lease, Right-of-Use Asset	$ 22.4	$ 28.6
Operating Lease, Cost	8.3	9.4
Operating Lease, Payments	8.3	9.4
Residual Value of Leased Asset	3.0
Other Current Liabilities
Lessee, Lease, Description [Line Items]
Operating Lease, Liability, Current	5.9	6.9
Other Noncurrent Liabilities
Lessee, Lease, Description [Line Items]
Operating Lease, Liability, Noncurrent	16.5	21.8
Other Liabilities
Lessee, Lease, Description [Line Items]
Operating Lease, Liability	$ 22.4	$ 28.7
Vehicles and Equipment [Member]
Lessee, Lease, Description [Line Items]
Operating Lease, Weighted Average Remaining Lease Term	3 years	4 years
Operating Lease, Weighted Average Discount Rate, Percent	3.10%	3.70%
Land and Other [Member]
Lessee, Lease, Description [Line Items]
Operating Lease, Weighted Average Remaining Lease Term	27 years	28 years
Operating Lease, Weighted Average Discount Rate, Percent	4.10%	4.10%